This invention relates to a method and composition for opening naphthenic rings of naphthenic ring-containing compounds such as distillate. In particular, this invention relates to the use of a catalyst composition comprising a combination of a naphthene ring isomerizing catalyst, preferably containing Pt and/or Pd in an amount effective to isomerize a C6 naphthene ring compound to a C5 naphthene ring compound, and a naphthene ring opening catalyst, preferably containing Ir in an amount effective to ring open naphthene ring compounds.
There is an increasing demand for hydrocarbons boiling in the distillate boiling point range (xe2x80x9cdistillatexe2x80x9d). Distillates typically contain paraffins, naphthenes, and aromatics. For fuel quality parameters such as cetane number, gravity and emissions, paraffins are the most desirable components, followed by naphthenes, followed by aromatics. The least desirable are multi-ring aromatic compounds. There is also an increasing demand for paraffinic solvents arising from their low toxicity and biodegradability. Consequently, it is desirable to reduce the cyclic compound content of hydrocarbon solvent blends, in general, and to convert naphthenes to paraffins, in particular. The general process of converting naphthenes to paraffins is referred to herein as ring opening.
Refinery processes that produce distillate fuels often have a limited capability to produce high quality and yields of distillate fuel. For example, conventional hydrogenation processes saturate aromatic rings to form naphthenes, thereby increasing the cetane number and increasing the API gravity (i.e., lowering the density). However, single ring and multi-ring naphthenes have generally lower cetane values and are denser than paraffins having substantially the same number of carbon atoms. The greater density of naphthenes results in reduced volume of the distillate fuel blend relative to compositions containing similar concentrations of paraffins instead of naphthenes. Hydrocracking catalysts, typically composed of hydrogenation metals supported on acidic supports, are also effective for aromatics hydrogenation and for ring opening by cracking. However, cracking tends to make lower boiling point products, including a significant quantity of undesired gas by-products, which lowers the overall boiling range and limits the volume of final distillate product. In fact, hydrocracking products generally do not contain more distillate boiling range paraffins than the hydrocracking feeds. Moreover, a significant portion of the total paraffin concentration in the final product of conventional hydrocracking processes, including gas by-products, are relatively low molecular weight compounds that are outside the distillate boiling range. Thus, the apparent increase in distillate boiling range paraffins and improved distillate fuel quality may result primarily from a combination of the hydrogenation of aromatics and a concentration of paraffins in a reduced volume of distillate product, the latter arising from removing the undesired paraffin gas by-product, i.e., the low boiling point paraffin gas components.
There is therefore a need for selective ring opening processes for converting single and multi-ring aromatic species, including alkyl functionalized derivatives thereof, into distillate boiling range paraffins without producing a significant amount of undesirable low boiling point saturated species. Selectivity for ring opening is related to the propensity for cleavage of a ring bond which results in product molecules having an equivalent number of carbon atoms and at least one less ring than the original molecule, rather than cleavage of a bond which results in a product molecule having fewer carbons than the original molecule. A perfectly selective ring opening process would give only ring bond cleavage to produce molecules having an equivalent number of carbon atoms and at least one less ring than the original molecule. For example, from a hydrocarbon stream containing only single ring naphthenes of n number of carbon atoms, the product from perfect ring opening selectivity would contain only paraffins of n number of carbon atoms. Thus, the greater number of product molecules from a ring opening process having an equivalent number of carbon atoms and at least one less ring than the original molecule, the greater the selectivity for ring opening.
Conventional ring opening processes use a wide range of catalysts, including bifunctional metal hydrogenation-acidic catalysts. However, distillate quality may be improved by controlling paring isomenzations and subsequent dealkylations in order to limit the number of lower cetane, highly branched paraffins that may result from conventional ring opening.
Some conventional processes for forming an improved distillate employ Ir catalysts for opening naphthene ring compounds. Even though distillates such as diesel, jet fuel and heating oil contain at least about 20 vol. %, generally about 20 to about 40 vol. % of C6 naphthenes, the conventional processes open C6 naphthenes at low rates, if at all. This problem is exacerbated with hydrotreated distillates because they have a still greater concentration of C6 naphthenes. In order to overcome this problem of poor opening of C6 naphthene rings, U.S. Pat. No. 5,763,731 teaches using Ir along with at least one acidic co-catalyst, preferably a zeolite, to isomerize the C6 naphthene rings to C5 rings. However, since the resulting C5 ring structure will typically bear increased numbers of substituents, such as alkyl groups, this approach increases the volume of branched paraffins upon ring opening. In addition, the presence of an acidic co-catalyst has a tendency to isomerize any naturally present linear paraffin into a branched paraffin, often resulting in a ring-opened product that has an undesirably high concentration of branched paraffins. Moreover, the process results in increased light saturated gas production, particularly at high temperature.
Another conventional process, set forth in U.S. Pat. No. 5,811,624, uses Ir along with at least certain transition metals for isomerizing C6 naphthene rings to C5 naphthene rings, with the Ir component being particularly effective for opening the C5 naphthene rings. However, the product contains a significant concentration of branched paraffins, which leads to a lower product cetane number.
There is still a need, therefore, for a ring opening process and catalyst which provide a much higher degree of linearity in the ring opened product.
In one embodiment, a ring opening catalyst system and process of this invention is provided to form a product higher in linear paraffin functionality compared to conventional ring opening catalysts and processes. The process accomplishes this by providing a naphthene ring opening catalyst system comprising a naphthene ring isomerizing catalyst containing a catalytically active naphthene ring isomerization metal supported on a first catalyst support in an amount effective to isomerize a C6 naphthene ring-containing compound to a C5 naphthene ring-containing compound. The catalyst system further comprises a naphthene ring opening catalyst containing a catalytically active naphthene ring opening metal, preferably a Group VIII such as Ir, Pt, Ru, Rh and more preferably Ir, supported on a second catalyst support in an amount effective to ring open a naphthene ring-containing compound.
The isomerizing catalyst and the ring opening catalyst may be mixed together or provided in a stacked bed arrangement. Preferably, the catalytically active isomerization metal is at least one of Pt and Pd. In one embodiment, the isomerizing catalyst contains from about 0.1 to about 2.0 wt. % Pt, Pd, or a combination thereof. Preferably, the ring opening catalyst contains from about 0.01 to about 0.5 wt. % Ir.
In one embodiment, the isomerization and ring opening metals are present at a weight ratio of 50-99 parts of isomerization metal to 50-1 parts of ring opening metal.
The first and second catalyst supports may be independently selected refractory inorganic oxides. Preferably, the refractory inorganic oxides are selected from the group consisting of alumina, silica, zirconia, titania, cluomia, zinc oxide, magnesia, thoria, boria, silica-alumina, silica-magnesia, chromia-alumina, alumina-boria, silica-zirconia, and combinations thereof. More preferably, the first substrate is alumina. Catalyst supports are also referred to herein as substrates.
In another preferred embodiment, either the naphthene ring isomerizing catalyst or the naphthene ring opening catalyst further comprise a hydrogenolysis suppressor selected from the group consisting of Group IB, IIB and IVA metals in an amount effective to moderate cracking of a naphthene ring-containing feed to form methane. Preferably, the hydrogenolysis suppressor is at least one of Cu, Ag, Au, Zn, and Sn. More preferably, the hydrogenolysis suppressor is Sn.
In yet another embodiment, the ring opening catalyst is supported on a modified refractory support having an effective amount of modifier for enhancing selectivity to linear paraffin functionality. Preferably, the modifier is at least one of Cs, Mg, Ca, and Ba. More preferably, the modifier is at least one of Ca, Mg, and Ba. Most preferably, the modifier is Mg.
In still another embodiment, the naphthene ring opening catalyst contains Ir and further contains at least one other Group VIII metal selected from Pt, Ru, and Rh. The Ir and the other, or xe2x80x9csecondxe2x80x9d, Group VIII metal are present in an amount effective for opening a naphthene ring at a tertiary carbon site. Desirably, Ir is present in a range of about 0.1 to about 2.0 wt. %, preferably in a range of about 0.3 to about 1.5 wt. %, more preferably in a range of about 0.5 to about 1.2 wt. %, and most preferably in a range of about 0.5 to about 1.0 wt. %, based on the weight of the ring opening catalyst. It is also desirable that the second Group VIII metal be present in a range of about 0.001 to about 2.0 wt. %, preferably in a range of about 0.005 to about 1.5 wt. %, more preferably in a range of about 0.007 to about 1.3 wt. %, and most preferably in a range of about 0.01 to about 1.0 wt. %, based on the weight of the ring opening catalyst.
In another embodiment, there is provided a process for opening naphthene rings of naphthene ring-containing compounds in a feed stream. The process comprises providing a naphthene ring-containing feed stream; and contacting the naphthene ring-containing feed stream with the naphthene ring opening catalyst system of the invention.
Ring opening may be carried out at a temperature ranging from about 150xc2x0 C. to about 400xc2x0 C.; a pressure ranging from about 100 to about 3,000 psig; a liquid hourly space velocity ranging from about 0.1 to about 10 V/V/Hr; and a hydrogen treat gas rate ranging from about 200 to about 10,000 standard cubic feet per barrel (SCF/B).
In another preferred embodiment, the feed stream is a petroleum feed stream which has a boiling point of from about 175xc2x0 C. to about 600xc2x0 C. For naphthenic rings containing at least one tertiary carbon site, the ring opening process desirably ring opens the naphthene ring at the tertiary carbon site, thereby forming a ring opened product having increased linear paraffin functionality relative to that of the feed stream. The ring opened product may be recovered and may be used xe2x80x9cas isxe2x80x9d and in blended form. Preferably, the ring opened product is blended with a petroleum stream having a boiling point of about 175xc2x0 C. to about 600xc2x0 C., wherein the blend has a cetane number of at least about 40. The petroleum stream may be, for example, one or more of diesel fuel, jet fuel, heating oil, vacuum gas oil, and light cycle oil.
Desirably, the naphthene ring-containing feed stream has a sulfur content of less than about 10 ppm, preferably less than about 1 ppm, more preferably less than about 0.1 ppm. It is also desirable that the naphthene ring-containing feed stream contains less than about 20 wt. % total aromatic compounds.
Also included as part of this invention are the products made by the stated processes.